Revised phase stability diagram of rare earth sesquioxides

Abstract

Below 2000°C rare earth sesquioxides (RESOX) have three crystal structures: hexagonal, cubic and monoclinic, designated as A, C and B respectively [1-3]. Early studies, based on low temperature (LT) synthesis, suggested that RESOX phase stability versus temperature is a function of the metallic ion radii (MIR). La2 O3 Ce2 O3 and Nd2 O3 with the highest MIR are A-type, while for Sm2 O3 , Eu2 O3 and Gd2 O3 with intermediate MIR the structure is C-type at LT and B-type at high temperature (HT) [1-3]. All other RESOX including Y2 O3 and Sc2 O3 were assumed to be cubic (C-type) at all temperatures below 2000°C. The transformation from LT cubic to high temperature (HT) monoclinic structure in Sm2 O3 , Eu2 O3 and Gd2 O3 is unusual and therefore Brauer [4] and Yokogawa et al. [5] suggested that the stable phase is monoclinic at all temperatures below 2000°C. To resolve the controversy, we have demonstrated that slowing down grain growth of Sm2 O3 and Gd2 O3 [9] prevented transition from C to B-types in the expected temperatures (1100 and 1300°C respectively). Hence, we suggest that the surface energy plays an important role in determining the structure of nanomaterials [6,7]. The monoclinic Sm2 O3 , Eu2 O3 and Gd2 O3 is the stable structure at all temperatures below 2000 °C when the grain size is large in the nanoscale.